A trio of scientists fascinated by the dynamics of small grains chased their curiosity into the desert. Their three years of observations show that barchans — dunes formed by wind from a single direction — are inherently unstable objects (see page 720).

Bruno Andreotti of the PMMH, a physics laboratory at the University of Paris VII, and his group decided to study barchans because they are model dunes, rather in the way that fruitflies are model organisms. However, there are very few places with the conditions needed to produce them; the closest is a region of the Sahara desert in southern Morocco. Hicham Elbelrhiti, a Moroccan grad student who spent half of his three-year PhD in the field, was accustomed to the sometimes harsh conditions, but Andreotti and Philippe Claudin took longer to adapt.

“After a few missions under canvas, we decided to rent a fisherman's house close to the sea,” says Andreotti. There they had the comfort of water and electricity, although no relief from the sand they were studying. The ubiquitous grains made life difficult for the cameras, computers and other instruments. “But the natural beauty of the landscape made us forget all difficulties,” says Andreotti.

That beauty slowly revealed itself over repeat viewing, in a process that Andreotti likens to the film Blow Up, in which repeat scrutiny of an enlarged negative gradually reveals clues. “As time goes by, your eye becomes sensitive to more and more details,” says Andreotti. “You can recognize the dunes in a familiar area, and their almost invisible changes.”

Dune roaming: Bruno Andreotti in the Sahara.

The truly subtle changes — in the undulations on the flanks of the dunes — became obvious from some observations that ran counter to previous models. Lab simulations based on these models indicated that barchans reproduce their shapes using scaling laws. “This was not the case in reality,” says Andreotti. The group's observations revealed other factors at work: the models didn't account for slight fluctuations in wind directions and the effects of storms on barchan formation.

Andreotti would like to use his observations to build models that take into account the effects of these instabilities. “We hope that such a model will be able to reproduce the structure of the dune field and will help us to understand what selects the size of dunes at a given place,” he says.

Andreotti and his group plan to extend their study to subaqueous ripples and dunes in rivers. “Are they different or do they result from the same instability mechanism?” asks Andreotti. The goal is to show that, when properly rescaled, dunes have the same physics in denser environments, such as water, and lighter ones, such as Mars.

The group is also collaborating with some Algerian physicists to understand how multidirectional winds create more complex shapes, such as ‘star’ dunes. The physical origin of these large dunes is still not understood and Andreotti says that nobody has succeeded in reproducing them in small-scale experiments or in simulations. Perhaps more trips to the desert will help sift through those sands.